Field of the Invention
The present invention relates to an image processing device that synthesizes a plurality of images, a method of controlling the same, an image pickup apparatus, and a storage medium.
Description of the Related Art
In general, in an image pickup apparatus, such as an electronic camera, a solid-state image pickup device (hereafter simply referred to as the image pickup device), such as a CCD or CMOS image sensor, is used. In this image pickup apparatus, various noises which cause degradation of the quality of an image are generated during the process of converting an optical image to an electric signal (analog signal) by the image pickup device. Examples of typical noise include reset noise generated in pixels and circuits for reading out signals from the pixels, and random noise, such as dark current generated in a pixel area, which varies whenever an image pickup operation is performed.
Further, within or in the vicinity of the body of the image pickup apparatus, there are provided components which can be noise sources that periodically change the power supply voltage of the image pickup device. If these noise sources operate during the signal read-out operation performed by the image pickup device, the power supply voltage supplied to the image pickup device is varied by the power supply variation caused by the noise sources and electromagnetic waves generated from the same.
In general, it is known that these noise sources are each driven at a frequency specific to each component or part thereof, and hence the power supply voltage of the image pickup device is periodically varied, and the variation in power supply voltage appears on an image as periodic horizontal stripe pattern noise.
By the way, there has been developed an image pickup apparatus that performs a high-speed read-out operation which enables a plurality of image pickup operations and signal processing operations to be performed within a short time period. In such an image pickup apparatus, one image is generated by performing addition processing for adding a plurality of images obtained by performing a plurality of image pickup operations at high speed to thereby average random noise, such as dark noise, included in each image. Therefore, compared with an image obtained by one image pickup operation, a synthesized image which is finally obtained is reduced in random noise, and is thereby improved in image quality.
On the other hand, the horizontal stripe pattern noise caused by a noise source which periodically changes the power supply voltage is sometimes increased in a synthesized image depending on a position on each image where the horizontal stripe pattern noise is generated before being subjected to addition processing.
To solve the problem of the horizontal stripe pattern noise caused by a noise source which periodically changes the power supply voltage, there has been proposed an image pickup apparatus that reduces the horizontal stripe pattern noise by detecting the frequency of the noise source, and changing a drive frequency of a drive signal for driving the image pickup device, according to the detected frequency (see Japanese Patent Laid-Open Publication No. 2010-141799).
However, in the image pickup apparatus disclosed in Japanese Patent Laid-Open Publication No. 2010-141799, since the drive frequency of the image pickup device is changed according to the frequency of the noise source, if a plurality of noise sources exist, it is necessary to change the drive frequency of the image pickup device on a noise source-by-noise source basis. This makes it difficult to change the drive frequency of the image pickup device when a plurality of noise sources exist, and in this case, the horizontal stripe pattern noise cannot be reduced.